Transformer wire coating compositions containing a three component resin system consisting of polyvinylacetal, phenol-aldehyde and epoxy resins are disclosed in U.S. patent applications Ser. No. 889,889; now U.S. Pat. Nos. 4,215,174 970,249; allowed and 075,077 now U.S. Pat. No. 4,254,231 (Flowers et al). The use of the epoxy resin in the wire coating composition allows solids contents of from 28 to 30% to be applied at room temperature and 36% solids to be applied at about 55.degree. C. The epoxy, when in the low molecular weight liquid form, improves the rheological properties of the coating during application. Viscosities between 1,000 and 10,000 centipoise are attained both at room temperature and at 55.degree. C. The liquid epoxy is also necessary in the 100% solid powder forms of these compositions in order to have the coating coalesce and flow out uniformly. The improved flow-out properties of the three component compositions allows the solvent quantity to be greatly reduced and even eliminated in the case of the powder resulting in both ecological and economical benefits.
The presence of the epoxy resins in the insulating coating compositions compromises some of the physical properties of the coating to the extent that optimum properties can be obtained only within narrow operating conditions.
U.S. Pat. No. 3,239,598 also discloses an insulated wire coating which may include an epoxy resin. Re-issue Pat. No. 25,625 discloses a structural adhesive composition which may also contain an epoxy resin. Both of these patents, however, are concerned with combinations of the components in resin systems which are outside that area disclosed within this application for resin systems as usable as wire coatings.
U.S. Pat. No. 3,412,354 discloses the use of a solution of a polyisocyanate or blocked polyisocyanate used as a dipping solution for transformer coils wound with adhesive overcoated wires which result in a rigid structure when heat treated. The instant invention distinguishes over the teachings of this patent by the addition of blocked polyisocyanates to a three component wire coating composition within an operable range of isocyanate groups in the composition.
The purpose of this invention, therefore, is to disclose methods and materials for increasing the range of operating conditions at which the three component wire coating compositions can be applied for the production of usable wire insulating coatings.